Underwater communication of data is often difficult. Underwater cabled data-communications connections are costly and are known to suffer from a variety of reliability problems, such as failure of cables due to material fatigue, excessive torque on connectors, and accidental entanglement by fishing boats and other vessels. Hydroacoustic data transmission is in many cases a preferred alternative. Data can be transmitted in to an observer either as raw measurement samples, or as pre-processed data. Instrument status signals can also be transmitted. Deployment of hydroacoustic transceivers is much less expensive and more readily accomplished than the deployment of cabled systems, particularly in the case of multiple transceiver units that are networked together. Several significant application areas are: environmental monitoring of underwater parameters and contamination, communication with underwater robotic vehicles, and control of underwater industrial apparatus. Since real-time data transfer is required in many underwater applications, it is important that an underwater data communications system provide high data rates with a high degree of reliability.
Underwater data communications systems that operate acoustically are typically limited to data rates of less than 50 bits per second (bps), and are characterized by low reliability, due to an interference phenomenon called "multipath", also known as "acoustic reverberation". An acoustic signal commonly traverses many paths as it travels towards a receiver. Multiple propagation paths can be caused by reflections from surfaces in the environment, for example. Some of these paths are longer than others. Therefore, since each version of the signal travels at the same speed, some versions of the signal will arrive after other versions of the signal. Sometimes the delayed signals will interfere with more prompt signals as they arrive at the receiver, causing signal degradation.
The multipath time-delay spread is the time that elapses between the moment that the earliest version of a transmitted signal arrives at a receiver, and the moment that the latest version of the signal arrives at the receiver.
Of particular importance in underwater communications are reflections of signals off the surface of the water, i.e., at the water-air interface. These reflected signals are ubiquitous, and have changing amplitudes and delays. The amplitudes of the reflected signals are sometimes nearly equal to the amplitude of the direct signal.
To understand multipath interference effects, it will first be necessary to explain the term "symbol". One or more symbols can be combined to form a message that conveys meaning. Each symbol must be uniquely recognizable, and is selected from a set of possible symbols, referred to as a symbol alphabet. For example, the letters "a", "b", and "c" are symbols from the English alphabet The numbers "0" and "1" are symbols of the binary number system. It is possible to represent symbols from a first alphabet with symbols from a second alphabet, such as representing the letter "a" by the combination of binary digits "00010010". Alternatively, the binary digit "0" can be represented by the combination of letters "zero".
To be transmitted via a transmission medium, a symbol must be represented as at least a portion of a signal waveform. The portion of a transmitted signal waveform corresponding to a symbol waveform is demodulated in the receiver over a period of time called a coherent integration interval, given correct synchronization. Correct synchronization requires that integration of the signal waveform begin at the actual beginning of the portion of the signal waveform that corresponds to a symbol. In the absence of correct synchronization, the symbol waveforms and therefore the meaning of the message will be misinterpreted.
Consider the case of a message transmitted as a binary data modulation waveform, wherein each message symbol consists of a single bit. When the multipath time-delay spread is longer than the temporal symbol separation, i.e., the time between symbols in the message, bits of the first received signal version overlap non-corresponding bits of the last received signal version. This phenomena is called intersymbol interference (ISI). In a typical underwater environment, the time-delay spread can often range from 0.1 milliseconds (ms) to 200 ms, for example. Since, in binary data modulation, data rate is the reciprocal (multiplicative inverse) of symbol duration, a time delay spread of 200 ms implies that data rates even much less than 5 bits per second (bps) will result in significant data errors due to intersymbol interference.
In addition to the intersymbol interference, some multipath reflections may have delay spreads that fall within the time between symbols. This type of multipath effect, referred to as intrasymbol interference, can cause significant degradation in the amplitude of the net received signal.
There are two types of intrasymbol interference. Reflections of significant amplitude having a delay that exceeds a half wavelength of the carrier frequency will cause periodic energy nulls in the frequency spectrum of the net received signal due to coherent cancellation at those frequencies. The bandwidth of the frequency nulls is inversely proportional to the delay of the signals that are interfering. This type of signal loss, commonly known as "frequency selective fading", undermines the reliability of the underwater communications link.
Another more serious type of intrasymbol interference occurs when a signal undergoes specular (mirror-like) reflection at the water surface at a grazing angle, i.e., the reflected signal path is substantially tangential to the surface. The water surface has a reflection coefficient that is approximately -1, meaning that the reflected signal experiences a 180.degree. phase shift with respect to the incident signal. The amplitude of the reflected signal is a function of the degree of smoothness of the water surface, where amplitude decreases with increasing surface roughness.
When the delay is a small fraction of the period of the carrier frequency, there will be signal cancellation at all signal frequencies from 0 Hz to the frequency at which the effect no longer occurs, which is a function of the geometry. This signal cancellation is called the "Lloyd mirror" effect. The Lloyd mirror effect is particularly a problem at shallow depths. Unlike "reverberation", which refers to interference due to many paths of significant delay, the Lloyd mirror effect is specifically due to the signals that have grazed the water surface, and have thereby experienced an amplitude inversion at the water surface. These inverted signals cancel the direct signal at the receiver. At shallow depths, as in shallow coastal waters, for example, and at moderate ranges, grazing signals are only slightly delayed with respect to the direct signals.
Both the Lloyd mirror effect and frequency selective fading assume a fixed geometric relationship of a transmitter, a reflecting surface, and a receiver, and result in signal loss over a range of frequencies. A received signal can also be adversely affected when frequency is held constant and the geometric relationship varies. As either the transmitter or receiver are moved in a direction that is normal to the reflecting surface, the received signal level varies as a function of position, the function being analogous to "grating lobes", a radiation pattern attributable to transducer behavior. Since the surface of a body of water is always in motion, the received signal amplitude is always modulated by the grating lobes.
Overcoming frequency selective fading is commonly accomplished using diversity methods that employ both spatial diversity and frequency diversity. Spatial diversity requires at least two receivers, each receiver having a spatially separate receive transducer so as to provide a different frequency selectivity pattern for each transducer. By contrast, frequency diversity receivers share a single broadband receive transducer, where the transmitted signal is duplicated and then transmitted on at least two carriers of respective frequencies that are separated by a frequency bandwidth that is larger than the frequency range of a frequency null.
The diversity receiver unit includes a plurality of receivers, each tuned to one of the carrier frequencies. The receiver outputs of either method fade independently, and are advantageously combined in one of several known ways to compensate for such independent fading. However, since this method employs a plurality of independent receivers, it can be quite costly to implement.
Another approach to overcoming the Lloyd mirror effect involves the use of a carrier of sufficiently high frequency to transform interference due to the Lloyd mirror effect into interference due to frequency selective fading. Nevertheless, diversity must still be employed to overcome the resulting interference.
Regarding intersymbol interference, to be distinguished from intrasymbol interference discussed above, there are known methods for mitigating intersymbol interference due to multipath effects, while also providing high data rates. To exploit the fact that signals having the longest delays often arrive from angles far from the central axis of the transducer, a first such method employs highly directional line-of-sight data links having high gain acoustic transducers. One problem with this method is that to obtain high transducer gain, large transducer arrays which are difficult to deploy and stabilize are required. Also, each transducer must be carefully oriented. Such transducer arrays are therefore complicated and expensive to install and to move.
A second method employs echo canceling techniques implemented with adaptive filters. At high data rates, the high expense and demanding computational requirements of adaptive filters is prohibitive in the dynamic environment of underwater acoustic communications.
A third method is to channelize the transmitted waveform into multiple channels, each channel being of different carrier frequency and of lower bandwidth (and therefore of longer symbol duration) than the single-channel transmitted waveform. Each channel is then received independently. This approach is excessively costly because one independent receiver per channel is required.
A fourth and less conventional approach is to use orthogonal signalling, wherein the waveform that represents each symbol has no projection on the respective waveform of any other symbol of the symbol alphabet from which the symbols of the message are selected. Consequently each symbol in the alphabet is more easily distinguished from other symbols in the alphabet than without the orthogonality property. If the temporal symbol duration of the orthogonal signal is made much longer than the multipath time delay spread, the effect of the multipath can be reduced. For example, one of many approaches includes the use of M-ary frequency shift keying (MFSK) modulation to encode the high-level symbol alphabet into one of M frequencies. However, in this context, orthogonal signaling requires a diversity receiver to overcome intrasymbol interference. Also, a lot of bandwidth is required to implement orthogonal signaling as compared with a communications channel without orthogonal signaling.
Generally, these approaches for eliminating intersymbol interference must also provide for diversity reception to mitigate intrasymbol interference, and consequently must operate duplicate receivers.